Chi-Yang Chao

Tuning perovskite morphology by polymer additive for high efficiency solar cell.

Solution processable planar heterojunction perovskite solar cell is a very promising new technology for low cost renewable energy. One of the most common cell structures is FTO/TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD/Au. The main issues of this type of solar cell are the poor coverage and morphology control of the perovskite CH3NH3PbI3-xClx film on TiO2. For the first time, we demonstrate that the problems can be easily resolved by using a polymer additive in perovskite precursor solution during the film formation process. A 25% increase in power conversion efficiency at a value of 13.2% is achieved by adding 1 wt % of poly(ethylene glycol) in the perovskite layer using a 150 °C processed TiO2 nanoparticle layer. The morphology of this new perovskite was carefully studied by SEM, XRD, and AFM. The results reveal that the additive controls the size and aggregation of perovskite crystals and helps the formation of smooth film over TiO2 completely. Thus, the Voc and Jsc are greatly increased for a high efficiency solar cell. The amount of additive is optimized at 1 wt % due to its insulating characteristics. This research provides a facile way to fabricate a high efficiency perovskite solar cell by the low temperature solution process (<150 °C), which has the advancement of conserving energy over the traditional high temperature sintering TiO2 compact layer device.

Theoretical Study on the Correlation between Band Gap, Bandwidth, and Oscillator Strength in Fluorene-Based Donor−Acceptor Conjugated Copolymers

There is a growing interest in developing low-band gap conjugated polymers via synthesis of copolymers containing alternating units of different pi-electron-donating/accepting capabilities. In this study, electronic and optical properties of conjugated copolymers containing fluorene and thiophene/cyclopentadithiophene derivatives are determined using density function theory and semiempirical ZINDO calculations. A remarkable linear correlation is found between the amount of charge transfer between the donor-acceptor pair, the band gap, the bandwidth, and the oscillator strength of S(0)-->S(1) electronic transition (ground state to first excited state) of the copolymers. Strong pi-electron withdrawing substituents, such as dicyanoethenyl and carbonyl groups, on the thiophene moiety effectively reduce the band gap of the copolymers. However, the reduction of band gap is frequently accompanied by a linear reduction in bandwidths and in the oscillator strength of S(0)-->S(1) transition. For very strong pi-electron withdrawing thiophene derivatives, the occurrence of maximum oscillator strength may even shift from S(0)-->S(1) to S(0)-->S(n>1) (ground state to a higher excited state), giving a blue shift in maxima absorption peak and a red shoulder in the UV-vis spectra as reported in recent experimental measurements. Therefore, the achievement of low band gap for conjugated polymers with alternating arrangement of pi-electron-donating/accepting moieties may be achieved at a cost of lowering electron mobility and optical efficiency and sometimes a blue-shift in the major optical (UV-vis) absorption.

Drop-Shape Analysis of Receptor−Ligand Binding at the Oil/Water Interface

Drop-shape analysis was used to study the binding of streptavidin to biotin at the interface between water and a pendant chloroform droplet. Polyethylene oxide molecules were synthesized with a hydrophobic tail at one end of the molecule and a hydroxyl or biotin group at the other end. The interfacial tension of the water/chloroform interface was measured before and after addition of these amphiphiles to the chloroform phase and before and after addition of streptavidin to the aqueous phase. The hydroxyl-terminated amphiphiles eliminate nonspecific adsorption of the streptavidin to the interface, while streptavidin binds irreversibly to the biotin-terminated molecules. Molecular interactions within this bound layer were studied by measuring changes in the interfacial pressure as the layer is contracted and expanded by changing the volume of the chloroform droplet. A picture of the interfacial structure was obtained from quantitative comparisons between the experimental results and a molecular theory of protein binding to tethered ligands. These comparisons suggest that protein binding is controlled by the extension of the PEO tethers away from the interface.

Band gap engineering via controlling donor-acceptor compositions in conjugated copolymers.

Varying composition of π-donor/acceptor moieties has been considered as an effective strategy for fine-tuning of the electronic properties of D-A conjugated copolymers. In this study, the change of optoelectronic properties with the change of donor/acceptor ratios is investigated on the basis of first-principles density functional calculations. Copolymers containing moieties of similar π-electron donating and/or accepting capabilities, e.g., thiophene (T)-methoxythiophene (OT), exhibit a linear dependence of electronic properties (especially, HOMO/LUMO, band gap, and bandwidth) on the D/A content. In contrast, for strong D/A contrast systems, e.g., thiophene (T)-thienopyrazine (TP), the electronic properties vary nonlinearly with D/A compositions. However, when the block size of one parent monomer in a strong D/A contrast system is fixed, the variation of electronic properties shows a remarkable linear correlation against D/A compositions. We found that the deviation of electronic properties from a linear composition dependence is dominated by the strength of orbital interactions between D and A. Weak orbital interactions between D and A moieties tend to lead to a nonlinear composition dependence. Our results provide useful insights for band gap tuning through the adjustment of D/A compositions in D-A conjugated copolymers.

Band structure engineering for low band gap polymers containing thienopyrazine

In this research, we demonstrated that the energy levels, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and the optical absorptions of low band gap conjugated copolymers, consisting of 3-hexylthiophene (3HT) as electron-donating units and 2,3-diethylthieno[3,4-b]pyrazine (ETP) as electron-accepting units, could be systematically tuned by adjusting the composition and the geometric structure of the copolymers. Four new copolymers comprising of 3HT and ETP in different molar ratios (from 1 : 1 to 4 : 1), named P1–P4, were designed and synthesized via Suzuki coupling. The positions of the hexyl side chains on 3HT were varied to adjust the co-planarity of the copolymers. The LUMO, ranging from −2.94 eV to −3.11 eV, was lowered monotonically with increasing ETP content, and the break of co-planarity along the main chain showed a trivial effect on the LUMO. By contrast, the HOMO (−4.74 eV to −4.88 eV) was controlled by both the composition and the geometric structure of the copolymer. P2, having a twisted geometric structure, possessed a lower HOMO and a larger band gap compared to the planar P3. Bimodal optical absorptions with a relatively stronger absorption at long wavelengths were observed for all polymers which are due to intramolecular charge transfer. Optical band gaps in solution, ranging from 1.27 eV to 1.76 eV, decreased with increasing ETP content in the copolymer except P2 and the trend was consistent with the HOMO–LUMO gaps. The optical absorptions and the energy levels are further confirmed by theoretical calculations. Good co-planarity was also found to benefit the electrical conductivity of p-doped thin films. Our results suggested that tuning the composition and the geometric structure would be an effective molecular design strategy toward desired band structure for low band gap conjugated polymers based on thiophene and thienopyrazine derivatives.